The use of experimental animals in scientific research has enabled some of the most important breakthroughs in medical research (The use of non-human animals in research: a guide for scientists: The Royal Society, 2004, incorporated by reference herein). Further refinement of animal models through genetic manipulation is an important and powerful tool in research today. By transplanting human cells and tissues into genetically engineered mice these possibilities are taken to a new level. Humanized mouse models present opportunities to study whole cellular systems in an in vivo setting (Strom S C et al, Methods Mol Biol 640, 491-509 (2010); Kamimura H et al, Drug Metab Pharmacokinet 25, 223-235 (2010); Schultz L D et al, Nat Rev Immunol 7, 118-130, (2007); Katoh M et al, Toxicology 246, 9-17 (2008) all of which are incorporated by reference herein). One such humanized mouse model is a mouse engrafted with human hepatocytes.